Tubular radiation absorbing device for a solar power plant with reduced heat losses

ABSTRACT

The tubular radiation absorbing device ( 1 ) for solar thermal applications includes a central tube ( 3 ) made of chromium steel, particularly stainless steel; a glass tubular jacket ( 2 ) surrounding the central tube so as to form a ring-shaped space ( 6 ); and a barrier coating ( 4 ) on at least an interior side of the central tube ( 3 ), which is substantially impermeable to hydrogen and contains chromium oxide. The barrier coating ( 4 ) is provided by a process in which the central tube ( 3 ) is treated with steam containing free hydrogen at a temperature of 500° C. to 700° C.

CROSS-REFERENCE

The invention described and claimed hereinbelow is also described in German Patent Application 10 2005 057 277.4-15, filed Nov. 25, 2005 in Germany, which provides the basis for a claim of priority under 35 U.S.C. 119.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a tubular radiation absorbing device for solar thermal applications, especially for a parabolic trough collector in a solar power plant, which comprises a central tube made from a chromium steel, especially stainless steel, and a glass tubular jacket surrounding the central tube so as to form a ring-shaped space between the tubular jacket and the central tube.

2. Related Art

Tubular radiation absorbing devices or absorber pipes are used in parabolic trough collectors to utilize solar radiation. The solar radiation is concentrated by a tracking mirror on a tubular radiation absorbing device and converted into heat. The heat is conducted away by a heat-carrying medium passing through the tubular radiation absorbing device and is used directly as process heat or converted into electrical energy.

This sort of tubular radiation absorbing device typically comprises a coated central tube and a glass tubular jacket around it. The ring-shaped space between the tubes is evacuated. In operation a heat carrier fluid, especially an oil, is pumped through the central tube.

This sort of absorber tube is described, e.g., in DE 102 31 467 B4. A glass-metal transitional element is arranged at the free end of a glass tubular jacket. The central tube and the glass-metal transitional element are connected with each other so that they are slidable relative to each other in a longitudinal direction by means of at least one expansion compensating device.

Free hydrogen, which is dissolved in the heat carrier medium, is generated during aging of the heat carrier fluid. This hydrogen arrives in the evacuated ring-shaped space between the central tube and the glass tubular jacket by permeation through the central tube. The permeation rate increases with increasing operating temperature, which is between 300° C. and 400° C., so that the pressure in the ring-shaped space rises. This pressure increase leads to increased heat losses and to a reduced efficiency of the tubular radiation absorbing device.

Suitable measures must then be taken to maintain a vacuum in the ring-shaped space. One measure that is taken to remove hydrogen is to combine it with a suitable material.

Getter material, which combines with the hydrogen gas that penetrates through the central tube into the ring-shaped space, is arranged in the ring-shaped space to maintain the vacuum. When the capacity of the getter material is exhausted, the pressure rises in the ring-shaped space until the partial pressure of the free hydrogen in the ring-shaped space reaches equilibrium with the hydrogen dissolved in the heat carrier medium. The equilibration pressure of the hydrogen in the ring-shaped space amounts to between 0.3 mbar and 3 mbar in the known absorber tubes. There is an increase in heat conduction in the ring-shaped space because of the presence of hydrogen in it. The heat losses due to heat conduction are about five times higher compared to air, i.e. clearly higher than with an absorber tube that has not been evacuated.

A getter arrangement is described in WO 2004/063640 A1, in which a getter strip is arranged between the central tube and the tubular jacket in the ring-shaped space. This arrangement has the disadvantage that the strip is in a region, which can be exposed to direct radiation. The getter strip can be heated especially by radiation coming from the mirror that misses the central tube or strikes it but is largely reflected from it. Since the getter strip is nearly thermally isolated from the central tube and the tubular jacket in a vacuum, the temperature of the getter strip can vary greatly with the varying radiating conditions. Because the getter material with a predetermined loading degree has a temperature dependent equilibrium pressure (equilibrium between gas desorption and adsorption), temperature fluctuations of the getter material lead to undesirable pressure fluctuations. The temperature of the tubular jacket greatly increases after consumption of the getter material and the absorber tube becomes unusable.

A chromium oxide coating or layer has been provided on chromium-containing steel according to “Initial oxidation and chromium diffusion. I. Effects of surface working on 9-20%-Cr Steels” by Ostwald and Grabke, Corrosion Science 46, pp. 1113-1127 (2004) in order to protect the steel from a reactive environment. The chromium-containing steel is provided with a coating by means of an H₂—H₂O atmosphere, which comprises an inner layer of Cr₂O₃ and an outer layer of (Mn,Fe)Cr₂O₄ spinel.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a tubular radiation absorbing device that has lower heat losses than conventional tubular radiation absorbing devices of the prior art.

This object is attained by a tubular radiation absorbing device, in which the central tube has a barrier coating that is largely impermeable to hydrogen, at least one its interior side. This barrier coating contains chromium oxide, Cr₂O₃.

It has been surprisingly found that coatings containing chromium oxide largely prevent passage of hydrogen.

The hydrogen diffusion from the interior of the central tube to the ring-spaced space could be reduced by a factor of up to 50 by this barrier coating.

The coating having chromium oxide is obtained by treating the central tube comprising steel, especially stainless steel, in a process in which a surface layer of the central tube is converted into a coating containing chromium oxide.

The barrier coating has a preferred coating thickness of 0.5 μm to 10 μm. The barrier action of the barrier coating decreases when the coating thickness is smaller than the foregoing preferred coating thickness. Crack formation increases in coatings that are thicker than this preferred coating thickness due to temperature changes, so that the barrier action similarly decreases when the coating thickness is thicker than the foregoing preferred coating thickness.

The chromium oxide content of the barrier coating is preferably from 20 wt. % to 60 wt. %, especially 30 wt. % to 50 wt. %. The chromium oxide fraction is determined by the chromium content of the steel and the type and duration of the treatment of the central tube, as explained in connection with the claimed process. The barrier action for hydrogen is initiated at a chromium oxide content of 20 wt. %.

Preferably the central tube has an outer coating on its outside which contains chromium oxide.

However it is preferred that the thickness of the outer coating is less than the thickness of the barrier coating. This coating merely serves as an adherent layer for a subsequently applied selective thin layer. The thickness of the outer layer amounts to preferably less than 0.1 μm. It has been shown that a spinel layer, which has a rough surface and is porous, is formed on the upper surface of the chromium oxide coating with a layer thickness of greater than 0.1 μm. This spinel layer is not suitable to support a subsequently applied smooth selective thin layer. The spinel layer does not interfere with the interior barrier coating, so that greater thickness is possible.

The process for making a central tube from steel containing chromium, especially from chromium-nickel steel, comprises first prefabricating a central tube from the steel, especially stainless steel, and then subjecting this central tube to a steam oxidation, in which the central tube is treated with steam containing free hydrogen at temperatures of from 500° C. to 700° C. in order to provide a barrier coating that is largely impermeable to hydrogen, at least on the interior side of the central tube.

Preferably the ratio V_(A)=H₂/H₂O of the steam for treating the outer side of the central tube is greater than the ratio V₁=H₂/H₂O of the steam for treating the inner side of the central tube. The formation of the spinel layer on the outer side is avoided by these process steps.

A preferred ratio V_(A) is from 10 to 1000, while a preferred ratio V₁ is from 1 to 100. However in this case V_(A)≧V₁.

According to another embodiment the coating thickness on the outer side can be reduced so that the central tube is worked or process on its outside prior to the steam treatment, so that it has a roughness R_(a) less than 0.3. Preferably the roughness R_(a) is less than 0.25.

A polishing procedure can be performed on the outer side of the central tube in order to perform this treatment.

In this second embodiment however the use of different values for the ratios V_(A) and V₁ is not required, but of course could be considered as an aid.

BRIEF DESCRIPTION OF THE DRAWING

The objects, features and advantages of the invention will now be illustrated in more detail with the aid of the following description of the preferred embodiments, with reference to the accompanying FIGURES in which:

The sole FIGURE is a cutaway longitudinal cross-sectional view through a preferred embodiment of the tubular radiation absorbing device according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A tubular radiation absorbing device 1 for solar thermal applications is shown in the cross-sectional view in the sole FIGURE. The tubular radiation absorbing device 1 comprises a central tube 3 made of metal and, a glass tubular jacket 2 surrounding the central tube so that a ring-shaped space 6 is formed between the tubular jacket and the central tube.

A heat carrier medium, which contains free hydrogen, flows through the central tube 3, which is made of metal. The hydrogen can permeate metal and thus pass through the central tube 3 into the ring-shaped space 6. In order to prevent the free hydrogen from passing through the central tube 3, which e.g. is made from Chromium-Nickel-Molybdenum 17-12-2 Steel No. 1.4404, it is provided with a barrier coating 4 on its interior side, which contains Cr₂O₃.

The inner coating 4 has a thickness of e.g. 10 μm. The coating 4 comprises a first layer and a further or second layer applied to the first layer. The first layer contains 30% Cr₂O₃, from 15 to 18% NiO, and from 50 to 54% Fe₂O₃. The further or second layer is predominantly composed of Fe₂O₃, i.e. 98% Fe₂O₃. The chromium oxide content of the second layer is only about 1 to 2%. This second layer, which forms the spinel layer, still contains a small amount of nickel oxide.

The central tube 3 has an outer coating 5 on its outer side, which has a thickness of 0.05 μm. This coating 5 has no spinel layer.

The preparation of the oxide coatings 4, 5 takes place by means of a steam oxidation process according to the following parameters:

H₂/H₂O ratio for both coatings 4,5,

Outer surface of the central tube: polished, Ra<0.2 μm,

Temperature T=500° C., and

Treatment time: 5 hours.

PARTS LIST

-   1 tubular radiation absorbing device -   2 tubular jacket -   3 central tube -   4 barrier coating -   5 outer coating -   6 ring-shaped space

While the invention has been illustrated and described as embodied in a tubular radiation absorbing device for a solar power plant with reduced heat losses, it is not intended to be limited to the details shown, since various modifications and changes may be made without departing in any way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention.

What is claimed is new and is set forth in the following appended claims. 

1. A tubular radiation absorbing device (1) for solar thermal applications, especially for a parabolic trough collector in a solar power plant, said radiation absorbing device comprising a central tube (3) comprising steel, said steel including chromium; a tubular jacket (2) comprising glass and surrounding the central tube so as to form a ring-shaped space (6) between the tubular jacket and the central tube; and a barrier coating (4) on at least an interior side of the central tube (3), wherein said barrier coating (4) is substantially impermeable to hydrogen and contains chromium oxide.
 2. The tubular radiation absorbing device as defined in claim 1, wherein said steel comprises a stainless steel.
 3. The tubular radiation absorbing device as defined in claim 1, wherein said barrier coating (4) has a thickness of 0.5 μm to 10 μm.
 4. The tubular radiation absorbing device as defined in claim 1, wherein said barrier coating (4) contains from 20 wt. % to 60 wt. % of said chromium oxide.
 5. The tubular radiation absorbing device as defined in claim 1, further comprising an outer coating (5) on an outer side of said central tube (3), and wherein said outer coating comprises said chromium oxide.
 6. The tubular radiation absorbing device as defined in claim 4, wherein said outer coating (5) has a thickness that is smaller than a thickness of said barrier layer.
 7. The tubular radiation absorbing device as defined in claim 5, wherein said thickness of said outer coating (5) is less than or equal to 0.1 μm.
 8. A process for making a central tube (3) of a tubular radiation absorbing device (1) for solar thermal applications, said process comprising the steps of: a) prefabricating a central tube (3) made of chromium-containing steel; and b) treating at least an interior side of the central tube (3) with free-hydrogen-containing steam at a temperature of from 500° C. to 700° C. in order to provide at least said interior side of said central tube with a barrier coating (4), wherein said free-hydrogen-containing steam comprises water and free hydrogen and said barrier coating (4) contains chromium oxide and is substantially impermeable to said free hydrogen.
 9. The process as defined in claim 8, wherein said chromium-containing steel comprises stainless steel.
 10. The process as defined in claim 8, wherein an outer side of the central tube (3) is treated with another free-hydrogen-containing steam containing said water and said free hydrogen in a ratio (V_(A)) of said free hydrogen to said water that is greater than a ratio (V₁) of said free hydrogen to said water in said free-hydrogen-containing steam that treats said interior side of said central tube (3).
 11. The process as defined in claim 10, wherein said ratio (V_(A)) of said free hydrogen to said water in said another free-hydrogen-containing steam is from 10 to 1000, said ratio (V₁) of said free hydrogen to said water in said free-hydrogen-containing steam is from 1 to 100, and said ratio (V_(A)) of said free hydrogen to said water in said another free-hydrogen-containing steam is greater than or equal to ten times said ratio (V₁) of said free hydrogen to said water in said free-hydrogen-containing steam.
 12. The process as defined in claim 8, further comprising working said central tube (3) on an outer side of the central tube so that surfaces of said outer side have a surface roughness (R_(A)) less than 0.3 prior to treating with said steam.
 13. The process as defined in claim 12, wherein said working comprises polishing.
 14. The process as defined in claim 8, further comprising working said central tube (3) on an outer side of the central tube so that surfaces of said outer side have a surface roughness (R_(A)) less than 0.25 prior to treating with said steam.
 15. The process as defined in claim 14, wherein said working comprises polishing. 